This invention relates to transconductor circuits, and more particularly relates to improved circuits for biasing transconductor circuits.
A transconductor circuit is a voltage controlled current source. The proportionality factor between the input voltage and the output current is referred to as the transconductance. Such circuits have general applicability, and in particular are used in filter circuits, multipliers and oscillators.
In some applications of the transconductor circuit, biasing requirements can be quite stringent. For example, in differential continuous-time filter circuits the common-mode bandwidth may be required to be larger than the signal bandwidth, in order to be practically useable. However, in such an application using another amplifier to set a common-mode reference can cause the circuit to have multiple dominant poles. Therefore, stabilizing such a transconductor circuit is difficult.
The problem is that in such transconductor circuits, common-mode feedback is required to enable correct functioning of the circuit, i.e., to maintain the common-mode voltage substantially at the level of the external reference. However, the signal path for this feedback originates at the output nodes of the transconductor circuit and ends where the loop is closed. Further, -a separate external reference voltage is required to set the direct current (D.C.) reference of the two output nodes, since a purely differential circuit merely defines one node with respect to the other. In order to be able to use, practically, these absolute output voltages, an external reference, e.g., with respect to ground, is required. Thus, combining these two requirements gives rise to the problems in stabilization discussed above.
Therefore, it would be desirable to be able to bias transconductor circuits while meeting strict performance requirements and circuit stability.
In accordance with the present invention there is provided a transconductor circuit, including a differential transconductor amplifier circuit. The transconductor circuit includes an input pair of transistors adapted to receive a differential input voltage, as well as a pair of output terminals adapted to provide a differential output current. A second pair of transistors provides current to the input pair of transistors. A floating voltage circuit is adapted to generate a floating voltage corresponding to a common-mode voltage of the differential output nodes and to control the second pair of transistors in response to the floating voltage to stabilize the common-mode voltage of the differential transconductor amplifier circuit.